In the science of ballistics, shock waves emanating from a projectile in flight and traveling faster than the speed of sound, interfere with and break-up the flow of air close to the aft or back end of the projectile. The disruption of air flow affects the flight of the projectile. In order to compensate for, or overcome such perceived interference and to impart spin to a projectile fired from a non-rifled or smooth bore system, the projectile is manufactured to include a boom or extension which provides distance between the nose and fins. In effect, the boom ensures that the fins, which might extend beyond the diameter of the body of the projectile, will contact intact air flow.
Alternatively, the projectile may have expanding fins. In such a case, the fins are hinged and spring loaded to the body of the projectile so that as the projectile exits the bore of a cannon on firing, and the fins expand beyond the caliber or diameter of the body of the projectile to engage intact air flow causing the projectile to spin.
U.S. Pat. No. 5,498,160 to Farina et al., which is incorporated herein by reference, describes a training projectile adapted to fly with limited range. The projectile includes a main cylindrical body having a generally conical nose cone at the front end of the main body, and a tail portion extending from the rear end of the main body. The tail portion includes a flared member which flares outwardly from the rear end of the main body to a tail portion end and provides drag in flight to limit the length of the flight. The tail portion also includes means to impart spin to the projectile, which means includes a plurality of slots in the flared member that are disposed at an angle relative to the longitudinal axis to impart spin to the projectile.
U.S. Pat. No. 5,328,130 to Gilman et al., which is incorporated herein by reference, describes an aft section connected to the nose of a projectile for imparting spin to the projectile. The aft section has two or more coaxial, adjacent, and integrally connected cylindrical segments of different diameters. The segment having the larger diameter is positioned most rearwardly of the projectile, relative to the nose of the projectile, and the periphery of this segment has circumferentially spaced angled slots for catching air moving past the projectile to spin the projectile. The segment with the smaller diameter attaches the cylindrical stabilizer to the aft end of the nose of the projectile and directs the flow of air to and through the angled slots of the segment having the larger diameter.
U.S. Pat. No. 5,070,791 to Dineen et al., which is incorporated herein by reference, describes a range limiting type of projectile used with a selective cartridge in a smooth bore gun. The projectile has a body member and a connecting tail cone which provides projectile roll, velocity decay and high drag to limit the projectile range. The tail cone has a conical outer surface that has a plurality of canted grooves.
The structures described above may have satisfied their intended purpose. However, there is still an unsatisfied need for an improved training projectile that simulates the velocity and flight characteristics of a service projectile with high drag control to limit the projectile flight range. More specifically, as the velocity of service projectiles increases, so must the velocity of the training projectiles, so as to simulate real flight characteristics. Training projectiles face strict flight limitations, as they are required to complete their descent within a specific range, for instance 8000 meters. As a result, an increase in velocity must be countered by a corresponding increase in drag.
Increased drag may be obtained, for instance, by increasing the size of the flared member in the Farina et al. patent, but not to exceed the inner diameter of the gun barrel within which the tail portion fits. Therefore, the amount of drag that can be imparted to the training projectile is limited by the physical dimensions of the gun, which, in certain systems, may not be sufficient to meet the flight range limitations imposed upon training projectiles.